This invention relates to a solid electrolytic capacitor and a manufacturing method thereof.
Following the digitization, miniaturization, and speed-up of electronic devices in recent years, there has been an increasing demand for capacitors for use in electronic devices, that are small in size, large in capacitance, and low in impedance in a high-frequency band.
As a capacitor that can respond to such a demand, there is a solid electrolytic capacitor having a conductive polymer layer (solid electrolyte layer).
This type of solid electrolytic capacitor is disclosed, for example, in Japanese Patent (JP-B) No. 2940059 (first related-art document). Referring to FIG. 1 illustrating a solid electrolytic capacitor similar to that disclosed in the first related-art document, the solid electrolytic capacitor comprises an anode (61) formed by a proximal portion 61 of a metal member 60 made of a valve metal, a dielectric layer (not illustrated) in the form of a valve metal oxide film formed at the surfaces of the anode 61, a cathode (80) in the form of a conductive polymer layer 80 formed on the anode 61 through the dielectric layer, and an anode lead (63) formed by a distal portion 63 of the metal member 60. In the figure, symbol 90 denotes a later-described resist.
The conductive polymer layer 80 is formed by immersing the proximal portion 61 of the metal member 60 in a monomer solution containing, for example, a derivative of pyrrole or a derivative of thiophene and a supporting electrolyte to thereby carry out electrolytic polymerization, chemical polymerization, or the like.
Herein, as disclosed in the first related-art document, the tape-like resist 90 is formed at an intermediate portion 62 between the proximal portion 61, which will serve as the anode 61, and the distal portion 63, which will serve as the anode lead 63, of the metal member 60. The resist 90 serves to prevent penetration of the monomer solution to the distal portion 63 of the metal member 60 while the proximal portion 61 of the metal member 60 is immersed in the monomer solution. The reason for preventing the penetration of the monomer solution to the distal portion 63 is to prevent occurrence of a leakage current between the cathode 80 and the anode lead 63 due to formation of a film of a conductive polymer contained in the monomer solution on the surfaces of the anode lead 63 when the solid electrolytic capacitor is completed.
However, the metal member 60 has a roughened surface layer 60a in order to obtain a large capacitance by increasing the facing area between the anode 61 and the cathode 80. Therefore, there is a possibility that gaps exist between the roughened surface layer 60a at the intermediate portion 62 of the metal member 60 and the tape-like resist 90. Consequently, there is a possibility that when the proximal portion 61 of the metal member 60 is immersed in the monomer solution, the monomer solution penetrates to the distal portion 63 of the metal member 60.
On the other hand, Japanese Unexamined Patent Application Publication (JP-A) No. 2000-243665 (second related-art document) discloses a measure for preventing the penetration of a monomer solution more securely than that disclosed in the first related-art document. This measure is, namely, a forbidden band formed on the surfaces of an intermediate portion of a metal member between its proximal portion to serve as an anode and its distal portion to serve as an anode lead. As one example of this forbidden band, the second related-art document teaches a combination of a separation groove formed by partly removing a roughened surface layer of the metal member so as to expose the body of the metal member and a resist formed on the separation groove.
However, even with the combination of the separation groove and the resist disclosed in the second related-art document, there is a possibility that the monomer solution penetrates to the distal portion of the metal member through minute gaps between the surface of the separation groove and the resist.
Further, there is also a possibility that the monomer solution creeps up the surface of the resist to reach the distal portion of the metal member. The creeping-up of the monomer solution remarkably occurs when immersing the metal member in the monomer solution so as to exceed the proximal portion thereof.
Herein, if the separation groove and the resist are prolonged, it may be more difficult for the monomer solution to penetrate or creep up. However, as a result of prolonging the separation groove and the resist, the redundant intermediate portion that does not serve either as the anode or as the anode lead is enlarged in the metal member. This is not preferable because it leads to an unnecessary increase in size of a solid electrolytic capacitor, that is, to a reduction in volumetric efficiency of a solid electrolytic capacitor.